This invention relates to a method for preparing 2-(substituted)4- methylsulfonyl benzoic acids which are useful in the preparation of certain 2-(chloro, bromo or nitro)-4-methylsulfonyl-benzoyl -1,3-cyclohexanedione herbicides, such as those described in U.S. Pat. No. 4,946,981 and U.S. Pat. No. 5,006,158. The 2-(substituted)-4-methylsulfonyl benzoic acids may be converted to their acid chloride, as described in U.S. Pat. No. 5,008,448. The resulting acid chloride or cyanide can then be reacted with certain 1,3-cyclohexanediones according to the process of U.S. Pat. No. 4,695,673 or U.S. Patent No. 4,708,127.
U.S. Pat. No. 4,704,467 teaches a method of preparing a mercaptobenzoate of the formula ##STR1## wherein R is alkyl, R.sup.1 is alkyl or aryl and X is halogen or nitro, including reacting a compound of the formula ##STR2## wherein P, and X are as defined above, with a mercaptan having the formula R-SH in the presence of an inorganic base, a phase transfer catalyst and a non-polar aprotic solvent.
The mercaptobenzoate may then be converted to its corresponding alkylsulfonyl benzoic acid.
It has now been surprisingly found that reacting a 2-(substituted)-4-nitrobenzoic acid with sodium methyl sulfinate can generate a 2-(substituted)-4-methylsulfonyl benzoic acid. This reaction is more favorable than the procedure in U.S. Pat. No. 4,704,467 for two reasons. First, the instant invention may be carried out using the benzoic acid and not the benzoate, thus saving a hydrolysis step, (and usually an esterification step because the alkylbenzoate is typically prepared from the acid). Second, because the instant method produces the sulfone directly, no additional oxidation step is necessary to produce the same from the mercaptan.